This research is designed both to provide information about genetic and environmental influences on traits contributing to thermoregulation, and to evaluate the efficiency of different genetic designs for the study of quantitatively varying traits. Major experiments include diallel analysis of physiological and behavioral responses to cold in Mus musculus, where the cross of 4 inbred strains provides estimates of the relative influences of additive and dominant gene action, as well as maternal and other reciprocal effects, on expression of these traits. These results will be compared to those from parent-offspring comparisons in 2 heterogeneous stock, one of which was synthesized from the 4 strains used in the diallel. These designs will also be used to partition sources of covariance as an analysis of common genetic influences on a multiple trait complex, allowing examination of temperature regulation as a biological whole. In addition, artificial selection has resulted in lines of mice which are highly, divergent for nest-building, a thermoregulatory trait. The underlying mechanisms and characters associated with this important behavior are being investigated.